1. Field of the Invention
The present invention relates to a master cylinder. The master cylinder is suitable for but not limited to use in a brake apparatus for an automotive vehicle.
2. Description of the Related Art
One type of master cylinder which has been proposed in the past has a seal cup secured inside a cylinder bore of a cylinder housing. A piston which passes through the seal cup is disposed inside the cylinder housing so as to be able to slide in the axial direction of the piston. When the piston is in a retracted position, a pressure chamber which is formed by the cylinder housing, the seal cup, and the piston communicates with an atmospheric pressure chamber through a connecting passage formed in the piston. When the piston moves by a prescribed amount from the retracted position in the forward direction, communication between the connecting passage and the atmospheric pressure chamber is cut off by the seal cup. Examples of such a master cylinder are disclosed in Japanese Utility Model Application Laid-Open (kokai) No. Hei 2-81268 and Japanese Utility Model Publication (kokoku) No. Hei 7-51416.
When the piston of such a master cylinder is in its retracted position, the connecting passage formed in the piston communicates between the pressure chamber and the atmospheric pressure chamber so that the pressure within the pressure chamber does not rise due to increases in temperature and other influences. In the master cylinder shown in Japanese Utility Model Application Laid-Open No. Hei 2-81268, the connecting passage in the piston is positioned such that when the piston is in its retracted position, the connecting passage communicates with the atmospheric pressure chamber to the rear of the seal cup so that the connecting passage is not shut off by the seal cup and members disposed to the rear of the seal cup. With such a structure, when the piston moves forwards from its retracted position, the idle portion of the stroke of the piston until the pressure in the pressure chamber increases is large, so it has the problem that the responsiveness at times of sudden operation of the master cylinder is poor.
With the master cylinder disclosed in Japanese Utility Model Publication No. Hei 7-51416, a seal portion which intimately elastically contacts the outer periphery of the piston is provided on the inner periphery of the end of an inner peripheral lip of the seal cup. A plurality of projections are provided on the low pressure side (the side of the atmospheric pressure chamber) of the seal portion. When the piston is in its retracted position, the connecting passage formed in the piston opens between the projections. In addition, in this master cylinder, the end of the inner peripheral lip of the seal cup contacts a groove surface (the end surface of a member which limits the forward movement of the seal cup), so free deformation of the inner peripheral lip in the radial direction of the piston is obstructed.
With such a structure, it is possible to decrease the above-described idle portion of the stroke (the portion of the stroke of the piston until the pressure in the pressure chamber is increased), so the responsiveness of the master cylinder at times of sudden operation can be improved. However, there are problems such as that a high degree of dimensional accuracy of each part is required to make the end of the inner peripheral lip of the seal cup contact the groove surface, it is necessary to provide a plurality of projections in the seal cup, formability of the seal cup becomes poor, costs increase, and free deformation of the inner peripheral lip in the radial direction of the piston is obstructed, so the sliding resistance at the start of movement of the piston becomes large.
The present invention was made in order to overcome the above-described problems. According to the present invention, a stepped hole is used as a connecting passage between a pressure chamber and an atmospheric pressure chamber of a master cylinder. An end of the stepped hole which opens onto the outer peripheral surface of a piston is given a large diameter, and an end of the stepped hole which always communicates with the pressure chamber is given a small diameter. The cross-sectional area in the small diameter portion of the stepped hole is selected so as to produce a throttling effect. In a preferred embodiment, the stepped hole comprises a small diameter portion which opens onto the pressure chamber at its inner end in the radial direction of the piston and a large diameter portion which is coaxially formed with respect to the small diameter portion and which opens onto the outer peripheral surface of the piston at its outer end in the radial direction of the piston. The stepped hole may be formed in the hollow end portion of the piston, with the small diameter portion of the stepped hole having an end opening onto the inner peripheral surface of the hollow end portion of the piston, and the large diameter portion of the stepped hole having an end opening onto the outer peripheral surface of the hollow end portion of the piston.
In a master cylinder according to the present invention, when the piston is in its retracted position, a pressure chamber and an atmospheric pressure chamber communicate through a stepped hole (a connecting passage) formed in the piston. The stepped hole formed in the piston opens onto the outer peripheral surface of the piston at its large diameter portion, so when the piston is in its retracted position, even when the center of the stepped hole is close to or overlaps the seal cup in the axial direction of the piston, the stepped hole communicates with the atmospheric pressure chamber with certainty. Therefore, in a state in which fluid communication between the pressure chamber and the atmospheric pressure chamber is guaranteed with certainty, the idle portion of the stroke of the piston until the pressure in the pressure chamber appropriately rises can be set to a small value.
In addition, the cross-sectional area of the passage in the small diameter portion of the stepped hole is set so as to produce a throttling effect, so when the piston rapidly moves from its retracted position in the direction of movement (forwards), even when the large diameter portion of the stepped hole communicates with the atmospheric pressure chamber, the pressure in the pressure chamber can be increased by the throttling effect. Accordingly, due to the synergistic effect of the ability to set the idle portion of the stroke of the piston to a small value and the ability to increase the pressure in the pressure chamber by the throttling effect, the responsiveness of the master cylinder during sudden operation can be increased.
In the master cylinder according to the present invention, the passage formed in the piston can be realized by a stepped hole, so it is unnecessary to modify the shape of the cylinder housing or to modify the shape of the seal cup. As a result, there are none of the various problems accompanying a modification of the shape of the cylinder housing or a modification of the shape of the seal cup, and the master cylinder can be easily and inexpensively realized. In addition, the inner peripheral lip of the seal cup is permitted to freely deform in the radial direction of the piston, so the sliding resistance of the piston and the seal cup can be reduced, and good operability can be maintained.
When carrying out the present invention, when the stepped hole formed in the piston comprises a small diameter portion which opens onto the pressure chamber at its inner end in the radial direction of the piston and a large diameter portion which is coaxially formed with the small diameter portion and which opens onto the outer periphery of the piston at its outer end in the radial direction of the piston, and particularly in the case in which the stepped hole is formed in a hollow end portion of the piston, the end of the small diameter portion of the stepped hole opens onto the inner peripheral surface of the hollow end portion of the piston, and the end of the large diameter portion of the stepped hole opens onto the outer peripheral surface of the hollow end portion of the piston, so it is possible to easily form the stepped hole by drilling or other method from the outer periphery of the piston, and the master cylinder can be inexpensively manufactured.